Cigarette smoke is known to contain a lot of harmful substances, among them carbon monoxide. Hence, there is a great interest in the industry to produce cigarettes, the smoke from which contains considerably fewer harmful substances. To reduce the amount of such substances, cigarettes are often provided with filters, typically made out of cellulose acetate. However, these filters are not able to reduce the amount of carbon monoxide in the cigarette smoke, since cellulose acetate cannot absorb carbon monoxide. Various suggestions for incorporating catalysts into the filter, to convert carbon monoxide into less harmful carbon dioxide, were not successful, partly for functional, partly for economic reasons.
Diluting the smoke associated with the cigarette, for example, by an airflow flowing through the perforation of the tipping paper, is also known. However, the amount of carbon monoxide in the cigarette smoke is reduced by this at the expense of diluting the taste of the substances defining the cigarette, and hence the taste sensation of the cigarette and customer acceptance are compromised.
The substances in cigarette smoke are determined by a method whereby the cigarettes are smoked under standardized conditions. Such a method is, for example, described in ISO 4387. In this, the cigarette is initially lit at the start of the first puff and then every minute, a puff is taken at the mouth end of the cigarette with a duration of 2 seconds and a volume of 35 cm3 with a sinusoidal puff profile. The puffs are repeated until the length of the cigarette falls below a length which is pre-defined in the standard. The smoke flowing out of the mouth end of the cigarette during the puffs is collected in a Cambridge Filter Pad and this filter is then chemically analyzed with respect to its content of various substances, for example nicotine. The gas phase flowing out of the mouth end of the cigarette during the puffs and through the Cambridge Filter Pad is collected and also chemically analyzed, for example to determine the quantity of carbon monoxide in the cigarette smoke.
During standardized smoking, the cigarette is thus under two different sets of flow conditions. During the puff there is a considerable pressure difference, typically in the range from 200 Pa to 1000 Pa between the inner side of the cigarette paper facing the tobacco and the outer side of the cigarette paper. Due to this pressure difference, air flows through the cigarette paper into the tobacco part of the cigarette and dilutes the smoke being generated during the puff During this phase, which lasts for 2 seconds per puff, the amount of dilution of the cigarette smoke is determined by the air permeability of the paper. The air permeability is measured according to ISO 2965 and defines the air volume per unit time, per unit area and per pressure difference unit which flows through the cigarette paper and hence has the unit cm3/(min cm2 kPa). It is often termed the CORESTA Unit (CU, CORESTA Unit) (1CU=1 cm3/(min cm2 kPa). With this parameter, the rod ventilation of a cigarette can be adjusted, that is, the air flow which flows through the cigarette paper into the cigarette during a puff at the cigarette. Typically, the air permeability of cigarette papers is in the range 0 CU to 200 CU, whereby the range from 20 CU to 120 CU is generally preferred.
In the period between the puffs, the cigarette smolders without any considerable pressure difference between the inside of the tobacco part of the cigarette and the surroundings, so that the gas transport is determined by the gas concentration difference between the tobacco part and the surroundings. Thereby carbon monoxide can also diffuse through the cigarette paper out of the tobacco part into the ambient air. In this phase, which lasts 58 seconds per puff according to the method described in ISO 4387, the diffusion capacity is the relevant parameter for the reduction of carbon monoxide.
The diffusion capacity is a transfer coefficient and describes the permeability of the cigarette paper for a gas flow that is caused by a concentration difference. More precisely, the diffusion capacity is the gas volume passing through the paper per unit time, per unit area and per concentration difference and hence has the unit cm3/(s cm2)=cm/s. The diffusion capacity of a cigarette paper for CO2 can, for example, be determined by the CO2 Diffusivity Meter from the company Sodim and is closely linked to the diffusion capacity of a cigarette paper for CO.
From the above considerations, it results that the diffusion capacity should have an independent, important significance for the carbon monoxide content in cigarette smoke and that the values for carbon monoxide in cigarette smoke should be able to be reduced by increasing the diffusion capacity. This is of particular importance with respect to the self-extinguishing cigarettes known in the prior art, for which comparably high values of carbon monoxide are observed. In such cigarettes, burn-retarding stripes are applied to the cigarette paper so that they self-extinguish in a standardized test (ISO 12863). This or a similar test is, for example, a part of the legal regulations in the USA, Canada, Australia and the European Union. The increased values of carbon monoxide are due to the fact that carbon monoxide can diffuse only to a very limited extent through the burn-retarding stripes out of the cigarette. It would be of great advantage to have cigarette papers available which compensate for this unwanted side effect.
In practice, however, it turns out to be very difficult to adjust the diffusion capacity independently of the air permeability of the paper in the paper production process. The air permeability by itself, however, is in most cases the subject of the paper specifications required by the cigarette manufacturers, so that—under this requirement—the diffusion capacity results practically from the paper production process and can only be varied within a very small range (compare also B.E.: The influence of the pore size distribution of cigarette paper on its diffusion constant and air permeability, SSPT17, 2005, CORESTA meeting, Stratford-upon-Avon, UK). This is because air permeability as well as diffusion capacity are determined by the porous structure of the cigarette paper, whereby there is a relationship between these parameters, which is given approximately by D*˜Z(1/2), whereby D* is the diffusion capacity and Z the air permeability. This relationship holds above all to a very good approximation if the air permeability of the paper is primarily adjusted by refining the pulp fibers.
From the prior art, various approaches are known for increasing the diffusion capacity of cigarette paper, for example by adding thermally unstable substances (WO 2012013334) or by selecting the mean size of the filler particles (EP 1450632, EP 1809128). Despite such attempts, there is still no instance of increasing the diffusion capacity for a given air permeability.